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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Tue, 22 Oct 2024 23:38:34 +1100
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On 22/10/2024 9:08 pm, Phil Hobbs wrote:
> john larkin <JL@gct.com> wrote:
>> On Mon, 21 Oct 2024 19:45:28 -0000 (UTC), Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> john larkin <jl@glen--canyon.com> wrote:
>>>> On Sun, 20 Oct 2024 19:25:08 -0000 (UTC), Cursitor Doom
>>>> <cd999666@notformail.com> wrote:
>>>>
>>>>> On Sun, 20 Oct 2024 11:42:27 -0700, john larkin wrote:
>>>>>
>>>>>> On Sun, 20 Oct 2024 14:25:33 -0400, bitrex <user@example.net> wrote:
>>>>>>
>>>>>>> On 10/20/2024 11:38 AM, john larkin wrote:
>>>>>>>> On Sun, 20 Oct 2024 14:15:19 +0100, piglet <erichpwagner@hotmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Yesterday in the thread "Random thoughts on sinewave oscillators" I
>>>>>>>>> posted a LT Spice wirelist showing amplitude levelling without fet,
>>>>>>>>> ntc thermistor or ptc filament instead using current steering long
>>>>>>>>> tail pair - kinda poor mans multiplier.
>>>>>>>>>
>>>>>>>>> Here is another alternative, using diode conductance bridge
>>>>>>>>> "variolosser".
>>>>>>>>>
>>>>>>>>> For the full 1960s vibe replace U2 U3 with bjt current sources.
>>>>>>>>
>>>>>>>> That's still limiting a Wein bridge, essentially a single-element
>>>>>>>> resonator.
>>>>>>>>
>>>>>>>> If you do the double-integrator type oscillator, you can do gain
>>>>>>>> control on the first integrator, and then the second one adds 6
>>>>>>>> dB/octave harmonic reduction.
>>>>>>>>
>>>>>>>> I think you can even get 12 dB/octave harmonic reduction if you do the
>>>>>>>> amplitude limiter right. 12*3 = 36!
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>> There's probably a way to finagle the 4th section of an LM13700 to apply
>>>>>>> a bias current inversely proportional to the output level into pin IB1,
>>>>>>> here:
>>>>>>>
>>>>>>> <https://imgur.com/a/xRVx2PC>
>>>>>>
>>>>>> At some point, opamp distortion and slew rate will usually dominate THD.
>>>>>> I'd guess that the LM13700 isn't very good.
>>>>>>
>>>>>> It might be OK as the small-influence AGC gain tweak, ahead of two good
>>>>>> integrating opamps.
>>>>>
>>>>> At the cost of adding another component, how about adding another opamp
>>>>> solely to regulate the output signal level? Say you want 2V p-p output,
>>>>> sample some of the output, rectify it and compare it to a reference level
>>>>> which is applied to the other opamp input.
>>>>> Just my 2c.
>>>>
>>>> You still need a multiplier of some sort to tweek loop gain. And
>>>> multipliers are nonlinear!

But it's precisely the non-linearity you want, where you want it, if you 
spend enough on a good quality multiplier.

>>> My vote would go to a comparator sensing the peak of the AC waveform, and
>>> reducing the DC bias current a smidge each time it fires.

That's actually unwise. Good quality Wein bridges get the harmonic 
content of their output about 120dB below the fundamental.

Comparators pull a spike of current from the supplies whenever they 
fire, which spread higher harmonics of the output frequency all around 
the circuit. Full wave rectifiers generate less harmonic noise in the 
power rails. You still need lots of power supply rejection, but lower 
levels of noise on the power rails mean you need less.

>> DC bias current of what?
> 
> An active device, specifically a bjt. You get transconductance proportional
> to collector current.

And the AD734 - and other well-designed Gilbert multipliers exploit that 
to the hilt.

-- 
Bill Sloman, Sydney